<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
<script>
  var a = 's';
  switch (a) {
    default:
      console.log(`a等于30`)

    case (31):
      console.log(`a比30大`);
      break
    case 91:
      console.log(`a比30小`);
      break
  }
</script>
</html>